The Commonwealth has admitted that its sanctions have had little effect on President Robert Mugabe.
Mugabe faces domestic and foreign pressure
Secretary General Don McKinnon told Reuters news agency that Mr Mugabe has not adopted political reforms despite Zimbabwe's suspension from the grouping of former British colonies.
Three African leaders are due to go to Harare to meet Mr Mugabe next week, amid intense speculation that they will urge him to step down.
"It's a classic case where we have failed. I claim we deserve an A minus for effort over Zimbabwe but about a D minus for achievement," Mr McKinnon said.
The Commonwealth said last year's elections were held in "a climate of fear", with opposition activists persecuted.
The US and the EU imposed travel restrictions against top Zimbabwean officials to protest against President Mugabe's re-election last year.
The sanctions are also linked to President Mugabe's policy to hand white-owned farms to landless black Zimbabweans.
The United States official responsible for African policy, Walter Kansteiner, has said targeted economic sanctions, though an important tool for influencing the Mugabe government, are not sufficient in themselves to bring about "needed change".
He has stressed the need for more international engagement, in particular from Zimbabwe's neighbours.
The Zimbabwe economy is in a severe recession, with unemployment at over 50% and inflation estimated at 228%.
The opposition blames Mr Mugabe's economic policy, while he says he is the victim of a western plot against him because of his redistribution of land from white to black farmers.
In a report published on Friday, campaign group Amnesty International said Zimbabwe's government had "introduced national legislation to silence criticism and opposition, perpetrate human rights violations and place the basic rights of the Zimbabwean people under siege".
Mr Mugabe recently told state television that he might be prepared to step down, having completed his land reform programme.
A spokesman for South African President Thabo Mbeki has, however, said he strongly rejected the notion that he would go to another country to bring about regime change.
Zimbabwe's opposition Movement for Democratic Change (MDC) has condemned the leaders of South Africa and Nigeria for being too soft on Mr Mugabe, after they favoured lifting the sanctions in March.
Mr Mbeki and his Nigerian counterpart Olusegun Obasanjo are two of the three Commonwealth leaders tasked with resolving the Zimbabwe crisis following last year's elections.
Zimbabwe's economy is in a severe recession
Despite the positions of Mr Mbeki and Mr Obasanjo, Zimbabwe's suspension was extended until December.
The MDC say that Mr Mugabe's resignation would be the first step to resolving the crisis but they insist that this must provide them with an equal role for them in a transitional government and fresh elections.
They fear that Mr Mugabe wants to hand power over to one of his close associates.
Amnesty says that the "state repression of fundamental human rights" in Zimbabwe is escalating.
"It is essential that the international community, particularly Southern
African governments, redouble their efforts to stress to the Zimbabwean
authorities that intimidation, arbitrary arrest and torture of government
critics is unacceptable," says its latest report.